The present invention provides a process for the preparation of compounds of the general formula: ##STR1## in which X is a hydroxyl group and X.sub.1 is a hydrogen atom or a hydroxyl group, and especially of luciferin, by the reaction of D-cysteine with 2-cyanomono- or -dihydroxybenzothiazole obtained from 2-chloromono- or -dimethoxybenzothiazole via a demethylation, wherein the demethylation is a